1. Field of the Invention
The present invention relates to a liquid crystal element having a composite layer which includes a liquid crystal material and a transparent substrate formed of, e.g., a resin.
2. Description of the Background Art
The liquid crystal element having a composite layer formed of a liquid crystal material and a transparent substrate formed of, e.g., a resin requires no polarizer or the like. Therefore, it has been expected to be applied to wide and bright liquid crystal display devices. Such liquid crystal elements are classified into the following three types, depending on the form of a composite layer.
Type 1 relates to an element including a pair of plates and a composite layer held between the plates. The composite layer has a large number of microcapsules formed of a resin material such as epoxy resin, and the microcapsules internally confine a liquid crystal material. The composite layer may include another resin in addition to the microcapsules. The liquid crystal element is prepared by mixing a liquid crystal material and a resin material, stirring the mixture at a high speed to form microcapsules, and filling a space between the pair of plates with the microcapsules.
Type 2 relates to an element including a pair of plates and a composite layer held between the plates. In the composite layer, a liquid crystal material and a resin material are phase-separated. As a resin material, for example, a photo-curing resin is used. This type of liquid crystal element is prepared by mixing a liquid crystal material, a photo-curing resin material and a photo-polymerization initiator, filling the resultant mixture into a space between the plates, and then curing the resin material by irradiating, e.g., ultraviolet rays to cause phase separation.
Type 2 can be sub-classified into three classes. The element of the first class includes a composite layer which is referred to as polymer dispersed liquid crystal (PDLC). In the composite layer, the liquid crystal material is dispersed in the form of liquid-cells in the resin material. The second class is included in the above PDLC except for that the liquid crystal is dispersed in the form of honeycomb structure in the resin material. This type of composite layer is prepared by increasing a rate of the liquid crystal material in the composite layer. The element of the third class includes a composite layer which is referred to as polymer network liquid crystal (PNLC). In this type of composite layer, the phase-separated resin material has a network structure called a three-dimensional network.
The element of the third type (type 3) includes a composite layer which is formed by impregnating a substrate having a large number of small pores with a liquid crystal material and is retained between a pair of plates. The substrates may be formed of resin, fibrous glass or the like.
For liquid crystal materials, it is known that a liquid crystal element exhibits a memory effect if it includes a composite layer formed of a liquid crystal material exhibiting cholesteric phase and a small amount of resin material added thereto. Such a liquid crystal element requires no memory elements such as TFT and MIM and has been paid attention to as an element for providing a high-precision display device.
Specifically, in a liquid crystal element including a composite layer, which is retained between two transparent plates and is formed of a liquid crystal material exhibiting a cholesteric phase such as a cholestric liquid crystal material and a chiral nematic liquid crystal material as well as a small amount of resin material added thereto, when a low pulse voltage is applied to the film through transparent conductive films positioned inside the plates, a focal conic state, i.e., a phenomenon that helical axes of liquid crystal molecules are irregularly directed occurs, so that the incident light beams are scattered, resulting in an opaque appearance of the liquid crystal. When a high pulse voltage is applied similarly, the helical axes of liquid crystal molecules are aligned vertically with respect to the plates and thus the planar state is obtained, so that the liquid crystal becomes transparent. These two states are maintained stably even after the voltage application is terminated. It has been considered that the bistability of these two states is achieved owing to the fact that the resin restricts movement of the liquid crystal.
The above liquid crystal element having the composite layer, which is formed of a liquid crystal material exhibiting a cholesteric phase and a transparent substrate such as resin, can also employ the above-described three modes of composite layers.
Regardless of the types of the composite layers, however, the conventional liquid crystal element, which includes the composite layer including the liquid crystal material exhibiting the cholesteric phase and the transparent substrate, cannot achieve a sufficient contrast. Therefore, development of liquid crystal elements showing a high contrast have been demanded.
Further, a liquid crystal material exhibiting a cholesteric phase selectively reflects light beams having a specific wavelength, which corresponds to the product of a helical pitch length and an average refractive index of the liquid crystal, to attain a colored transparent state, when it is in the planar state. Therefore, transparent-to-white display can be achieved by adjusting the wavelength of selective reflection of the liquid crystal to be, for example, in the infrared range. In this case, however, a transmittance of visual rays decreases in the planar state, resulting in a reduced contrast.